1. Field of The Invention
This invention relates to radio frequency (RF) amplifiers and, more particularly, to a pilot and input signal synchronization scheme for feed-forward RF amplifiers.
2. Description of Related Art
RF amplifiers often add undesired distortion to an input signal, creating an output RF signal on a main path comprising an amplified input signal and distortion, a significant component of which is intermodulation distortion (IMD). The distortion includes any undesired signals added to or affecting adversely the amplified input signal. IMDs of a signal arises from intermodulation of the frequency components in the signal by each other in a nonlinear system, producing waves having frequencies, among others, equal to the sums and differences of the frequency components of the original signal. Feed-forward correction is routinely deployed in modern RF amplifiers to reduce the distortion produced from the RF amplifier on the main signal path. The essence of the feed-forward correction is to isolate the distortion produced from the amplifier on a correction path so that at the final summing point, the distortion on the correction path cancels out the distortion of the RF signal on the main path.
Due to the unpredictability of input RF carrier pattern as well as the resultant location of the distortion, a known frequency component, i.e. the pilot, is injected in the main loop to mimic the distortion produced by the amplification process. In feed-forward amplifiers, the correction circuitry isolates the amplified pilot signal along with the distortion onto the correction path and manipulates the pilot signal and the distortion on the correction path to combine with and reduce the pilot and the distortion on the main signal path. The correction circuitry detects the pilot signal and attempts to cancel the pilot signal from the main signal path. In cancelling the pilot signal from the main signal path, the correction circuitry cancels the distortion.
There are two general types of pilot signals: continuous wave (CW) and spread spectrum (SS) pilot signals. The CW pilot is easy to detect and measure, but runs the risk of being overlaid by one of the input carriers. Therefore, it is advantageous to move the CW pilot outside of the band of operation. Otherwise, the pilot frequency should be constantly updated to find a quiet location in-band where there is no input carriers. For example, FIG. 1 shows the frequency response of an RF amplifier including the location of a CW pilot signal. The pilot signal can be near the lower edge of the operating band (e.g., pilot 1) or located near the upper edge of the band of operation (e.g., pilot 2). The pilot is positioned a spectral distance of xcex94f from an edge of the band of operation whose center frequency is f0. As mentioned above, the pilot signal can also be located somewhere within the band of operation of the RF amplifier. The electrical characteristics (e.g., amplitude, phase response, spectral content) of the pilot signal are known. It should be noted that although the pilot signal is shown as a single spectral component of a certain amplitude, the pilot signal can comprise a plurality of spectral components having various amplitudes. Furthermore, an SS pilot can be spread across the entire operating band for the RF amplifier. The SS pilot is harder to detect and measure, but it is immune to the input carrier placement when placed in-band.
FIG. 2 discloses typical feed-forward correction circuitry 10, which uses information obtained from the pilot signal to reduce distortion produced by RF amplifier 12. An input signal is applied to a splitter 14. The splitter 14 replicates the input signal on a main signal path 16 and a second path 18. The splitter 14 is part of a feed forward loop referred to as loop # 1, which in addition to the splitter 14, comprises gain and phase circuit 20, coupler 22, the RF amplifier 12, delay circuit 24 and couplers 26 and 28. The input signal on the main signal path 16 is applied to gain and phase circuit 20. The output of gain and phase circuit 20 and the pilot signal are applied to the coupler 22. Typically, the amplitude of the pilot signal is much less (e.g., 30 dB less) than the amplitude of the input signal so as not to create additional significant IMD components from the amplifier 12 due to the pilot signal. The output of coupler 22 is applied to the amplifier 12 whose output comprises the amplified input signal, the amplified pilot signal and distortion signals produced by the amplifier 12. A portion of the output of the amplifier 12 is obtained from the coupler 26 and is combined with a delayed version of the input signal (signal on path 18) at the coupler 28 via coupling path 30. The input signal on the path 18 has experienced sufficient delay provided by delay circuit 24, the delay of which is designed so that such signal experiences the same delay as the signal appearing at the coupler 28 via the path 30.
The gain and phase circuit 20 is controlled via control path 32 with two control signals to adjust the gain and phase of the input signal such that the input signal appearing at the coupler 28 via the path 30 is substantially the inverse (equal in amplitude but 180xc2x0 out of phase) of the delayed input signal at the coupler 28. The control signal appearing on the control path 32 of the gain and phase circuit 20 is derived from the signal at point A in a well known manner through the use of detection circuits. The detection circuits detect well known electrical signal characteristics such as amplitude, phase, and frequency of the signal. Therefore, the input signals applied to the coupler 28 cancel each other leaving at point A essentially the pilot signal and the distortion produced by the amplifier 12. Loop # 1 is thus a feed forward loop which serves to isolate at point A the pilot signal and distortion produced by the amplifier 12.
The signals appearing at point A (pilot signal and distortion signals) are fed to gain and phase circuit 34 whose output is fed to amplifier 36 whose output is applied to coupler 38. A portion of the output signals (input signal, pilot signal and distortion signals) of the amplifier 12 is fed to delay circuit 40 whose output is fed to the coupler 38. The delay circuit 40 is designed such that signals from the output of the amplifier 12 applied to the coupler 38 experience the same delay as the signals from the output of the amplifier 36 applied to the coupler 38.
Because the frequency, amplitude and other electrical characteristics of the pilot signal are known, pilot detect circuit 42 can use circuits such as a mixer connected to a log detector (or other well known detection circuits) to detect the pilot signal or a portion of the pilot signal via coupler 44. The pilot signal is used to obtain information about the distortion left at the final output. The information is obtained by detecting well known electrical signal characteristics of the pilot signal. In particular, the characteristics (e.g., amplitude, spectral content, phase response) of the pilot signal are known and thus when the pilot detect circuit 42 detects alterations to the pilot signal, detection circuit 42 will use such information to generate control signals onto path 46. The control signals on the path 46 cause the gain and phase circuit 34 to modify the pilot signal and distortion at point A such that the pilot signal and the distortion on the main path 16 at the coupler 38 is the inverse (equal in amplitude but 180xc2x0 out of phase) of the pilot signal and the distortion on the second path 18 at the coupler 38. The corresponding pilot signals and the distortion signals at the coupler 38 cancel each other at the coupler 38 essentially leaving the amplified version of the input signal at the output of the system. Therefore, loop # 2, which comprises the coupler 26, the coupler 28, the gain and phase circuit 34, the amplifier 36, the coupler 38 and the delay circuit 40 is a feed forward loop which uses the information obtained from the distorted pilot signal to cancel substantially the distortion produced by the amplifier 12.
In current systems, as the input signal power level is decreased, the IMD power levels produced also decrease, but the pilot power level remains constant. Since the pilot level does not decrease with the rest of the spectrum, it risks standing out as the highest distortion product, unless the correction circuitry 10 can further cancel the pilot signal at the coupler 38. For example, an RF amplifier can have an input signal range of 30 dB where the input signal has an absolute power level range from 2 dBm to xe2x88x9228 dBm. If the input signal is at 2 dBm and the pilot signal is at xe2x88x9228 dBm, a 30 dB input signal to pilot ratio is achieved at the input to the amplifier 12. If the input power level changes to 0 dBm, the pilot power level remains at xe2x88x9228 dBm, thereby the input signal to pilot signal ratio is reduced to 28 dB. At the low end of the input signal range (for example, when the input signal is down to xe2x88x9228 dBm), the pilot signal remains at xe2x88x9228 dBm and risks becoming a significant distortion component unless the correction circuitry 10 can significantly reduce the pilot at the output of the RF amplifier. Current systems use a constant power level for the pilot signal, for example xe2x88x9250 dBm. In determining the power level of the pilot, the pilot signal should not become a significant source of distortion at the low end of the input signal range (e.g. xe2x88x9228 dBm), and the pilot signal should be sufficient to cancel the distortion from the output of the amplifier 12 at the high end of the input signal range (e.g. 2 dBm). However, certain compromises are made in using a constant power level for the pilot signal. For example, at the high end of the input signal range, the correction circuitry has difficulty in reducing the distortion from the output the amplifier 12 because the distortion power level is generally higher with higher input signal power levels. At the low end of the input signal range, the pilot signal becomes a source of distortion.
The present invention involves a pilot adjusting system which adjusts the pilot signal relative to the input signal. For example, the pilot adjusting system detects the power level of the input signal on the signal path leading to an RF amplifier. In response to the power level of the input signal, the pilot adjusting system adjusts the power level of the pilot signal which is injected into the signal path prior to the RF amplifier. In certain embodiments, the pilot adjuster adjusts the pilot power level to maintain a desired input signal to pilot signal ratio at the input to the RF amplifier for the input signal range of the RF amplifier. As such, if the input signal power level drops 30 dB, the pilot adjuster reduces the power level of the pilot signal by 30 dB, thereby maintaining the input signal to pilot ratio throughout the input range of the RF amplifier. Other parameters of the pilot signal, such as phase and/or frequency, can be adjusted relative to other parameters for the input signal prior to the pilot signal being injected into the main signal path prior to the RF amplifier.